Electrostatic spray coating is an established art. In general coating material is projected toward an object to be coated in an atomized or particulate form from a dispensing device. The object to be coated is held at electrically ground potential and either just before, at, or just after being dispensed from the gun, the coating material is imparted an electrical charge so that it will be electrostatically attracted toward the object to be coated.
Because of the high voltage, certain safety precautions must be observed in the construction and operation of an electrostatic coating device. For example, when spraying many of the coating materials in use today, including the powders, a flammable atmosphere results in the area of the coating operation. If the electrostatic charging circuit associated with the spray guns is brought too closely to any grounded object, the possibility arises that a spark will jump between the high voltage circuit in the gun, and the grounded object. If there is sufficient energy in the arc thus produced, there is a possibility of igniting the flammable atmosphere in the coating area. The energy required for ignition may vary depending on the composition of the coating material, and the ratio of the material with respect to the air in the coating area. In order to reduce the amount of energy in a potential arc from the electrostatic charging system of the gun, high value resistors have been employed in the barrel of the gun. The resistors used in electrostatic spray guns operate to limit the current and thus lower electrical energy available to an arc. In order for the resistor to be effective however, the current must pass through it. Thus, current resulting from energy capacitively stored "downstream" of the resistor, is not limited by the resistor.
In general, previous designs of electrostatic guns incorporated the resistors in the barrel portion of the gun. Therefore, in electrostatic spray guns having a charging mechanism in the nozzle, energy was capacitively stored downstream of the resistor in the nozzle, and this energy was available to feed an arc. The amount of this capacitively stored energy increases as the square of the voltage. Therefore, guns of previous design had to be operated at lower voltages to result in safe energy storage levels downstream of the resistor. Lower operating voltages contribute to less than desirable coating characteristics and lower deposition efficiency.
The electrostatic spray gun comprised of the present invention has an improved high voltage charging circuit which results in safer operation without any appreciable degradation in efficiency while still allowing the use of a preferred electrostatic charging configuration. In the present design a second resistor is included in the nozzle portion of the gun so as to leave very little conductive material "downstream" of resistors.
The gun comprises a barrel portion with a high voltage electrical path in it with a resistor comprising part of the electrical path in the barrel. Attached to the barrel is a nozzle portion made from substantially non-conductive material, having a fluid passage ending in a discharge orifice, a high voltage electrical path therein and a thin wire-like electrode extending therefrom. The electrode is a conductive wire in contact with the electrical path in the nozzle and is made to have a small electrical capacitance. The electrode can be located close to or in the stream of fluid being discharged from the nozzle. The electrical path inside of the nozzle portion connects the electrode to the electrical path in the barrel and also comprises a resistor. This small resistor in the nozzle and the resistor in the barrel combine to effectively damp out or dissipate the stored energy resulting from the electrical circuit downstream of the large droping resistor in the barrel except for a small amount due to the electrode itself. Thus, it has been discovered that a smaller valued resistor can be used in the nozzle portion of the gun in conjunction with a high megohm resistor in the barrel portion of the gun to result in a safer gun at any given operating voltage, and a gun capable of use at higher voltages for any given safety margin.
The particular configuration of the gun facilitates ease of manufacture and assembly, good wear characteristics and constancy of the high voltage electrical characteristics of the gun.